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Scottish Settlements Urban and Rural Areas in Scotland

Annex A - The General Register Office for Scotland Postcode Database and the 2001 Census

Use of postcodes in the 2001 Census

5. The General Register Office for Scotland (GROS) will be grouping together postcodes to form Enumeration Districts, that is, workloads for enumerators of up to 400 households. GROS also plan to group postcodes to form Output Areas (OAs) which will be the lowest level geography used to produce statistical output from the Census. To avoid disclosure of statistics:

• each OA will have at least 20 households and 50 residents;
• each OA is wholly assigned to a ward, postcode sector, settlement and so on with 'nesting' as exact as possible; and
• there will be only one grouping of postcodes into OAs.

The last two conditions are to ensure that any slivers between slightly different areas contain at least one OA (and therefore at least the requisite number of households and residents to meet the disclosure control requirements)

6. The OAs are nested into a variety of higher geography areas to enable accurate estimates to be produced. However this can produce conflicts in which nesting within one type of area may have to be given priority over nesting within another. In creating OAs for 2001, GROS will give the following areas priority, in descending order:

• council area - to give exact statistics for administrative areas;
• settlement - to give exact statistics especially for the smaller settlements where, increasingly, rural policies may adopt a variety of cut-offs so as to include those with less than a given size as 'rural';
• 1991 OA - continuity at the detailed local level;
• postcode sector - for continuity with earlier Censuses; and
• 2001 electoral ward - users have expressed support for more prominence being given to these areas which may feature in statistics for the EU.

If other areas were added to this list, there would be too many constraints on the creation of OAs. Work is currently being done to check the impact of this order, particularly the effects on continuity of placing settlements above 1991 OAs.

7. The method of creating OAs is as follows. First, postcodes are grouped by common values of each of the 5 higher areas mentioned above. There is also a size restriction placed on the OAs. The target size is 50 households or 125 residents, with minimum and maximum threshold restrictions. The group is determined as being below-threshold, within-threshold or above threshold:

• less than 20 households/50 residents: this is where the higher areas have to be prioritised. The group is merged with a neighbouring group so that the resulting group is more likely to contain postcodes from more than one low priority area than from more than one high priority area. The group is re-assessed against the range;
• between 20-80 households/50-200 residents: the group becomes an OA; and
• above 80 households/200 residents: the group is split so as to create OAs that are geographically compact and have populations within range.